If you're an XNA developer that has been looking for a quick way to develop a 3D engine, look no further than the QuickStart Engine. The community designed it to help a programmer create a 3D game in XNA Game Studio as quickly as possible. It can be found over on Codeplex at http://www.codeplex.com/QuickStartEngine.
Video of v0.16 (they are on 0.178 now)
They have a ton of features already built in too. The water buoyancy and ping pong balls would be something I'd be able to play with all day.